The GroEL-like protein of B. pertussis, the major molecular chaperone produced by this organism, was purified. This protein was found to have the tetradecameric structure typical of the GroEL family of proteins and to contain epitopes similar to other members of this family, including a human GroEL-like protein. Active immunization of neonatal mice with the B. pertussis protein provided little protection against an aerosol challenge with B. pertussis. Antibodies to this protein were elicited in mice by a standard diphtheria-tetanus-pertussis (DTP) vaccine but not by an experimental acellular pertussis vaccine. Since the Bordetella GroEL-like protein was found to contain epitopes similar to those on the mammalian analog, the potential exists that vaccination with standard DTP vaccines may induce antibodies which react with the mammalian GroEL analog. Studies are continuing aimed at identifying the role this protein may play in the folding and assembly of pertussis proteins.